2. Following or opposing winds . In general, swell should 

 be forecast on the basis of preceding and current weather maps, 

 assuming that it travels through regions of calm. However, if a 

 prognosis of the weather situation or if a subsequent weather map 

 shows that the waves travel through regions where the wind has a 

 component parallel to the direction of progress, the forecast 

 should be modified by taking into account the effect of a follow- 

 ing or an opposing wind. 



The region of a following or an opposing wind has to be 

 considered as a second wind area, the boundaries of which have 

 to be selected as the boundaries of the region in which the 

 component , U' , of the wind parallel to the direction of progress 

 of the swell exceeds 6 knots . The reasons for this limitation 

 are that cross winds are not considered to influence the 

 swell and that the effect of very weak winds is negligible. The 

 wind velocity in the second wind area is obtained by estimate if 

 a prognosis is made or in the manner described above if a subse- 

 quent weather map is used. 



3. Effect of following or opposing winds . The effect of 

 the following or opposing wind on the wave height only has to be 

 determined, because it is assumed that the wave period is not in- 

 fluenced by these winds and that, consequently, the travel time 

 remains unaltered . Travel time and wave period at the end of 

 the distance of decay are therefore found by means of Plate VI 

 in the manner described above. 



In order to determine the v/ave height at the end of the dis- 

 tance of decay, H-^, j-r, , the auxiliary quantities listed in Table V 



47 



